1. Technical Field
The present application relates to an optical microphone which receives an acoustic wave propagating through a gas such as air, and converts the received acoustic wave into an electrical signal by use of light.
2. Description of the Related Art
Microphones have conventionally been known as devices for receiving an acoustic wave and converting it into an electrical signal. Many microphones, such as dynamic microphones and condenser microphones, have diaphragms. In these microphones, an acoustic wave is received as it vibrates a diaphragm, this vibration being taken out as an electrical signal. Since this type of microphone has a mechanically vibrating member, the characteristics of the mechanically vibrating member may be altered through repetitive use over a large number of times. Moreover, when the microphone is used for detecting a very intense acoustic wave, the vibrating member may possibly be destroyed.
In order to solve such problems of conventional microphones having a mechanically vibrating member, for example, Japanese Laid-Open. Patent Publication No. 8-265262 (hereinafter “Patent Document 1”) and Japanese Laid-Open Patent Publication No. 2009-085868 (hereinafter “Patent Document 2) disclose optical microphones which lack a mechanically vibrating member but detect an acoustic wave by utilizing light waves.
For example, Patent Document 1 discloses a method which modulates light with an acoustic wave, and detects the acoustic wave through detection of the modulated component of light. Specifically, as shown in FIG. 15, laser light which is shaped by using outgoing optics 101 is allowed to act on an acoustic wave 5 which propagates through the air, whereby diffracted light is generated. At this time, two diffracted light components having mutually inverted phases occur. After the diffracted light is adjusted through light-receiving optics 102, only one of the two diffracted light components is received by a photodiode 103 and converted into an electrical signal, whereby the acoustic wave 5 is detected.
Patent Document 2 discloses a method which allows an acoustic wave to propagate through a medium, and detects the acoustic wave through detection of changes in the optical characteristics of the medium. As shown in FIG. 16, an acoustic wave 5 propagating through the air is introduced through an aperture 201, and travels through an acoustic waveguide 202, at least a portion of whose wall surface is made of a photoacoustic propagation medium 203. The acoustic wave traveling through the acoustic waveguide 202 is led into the photoacoustic propagation medium 203, and propagates through the interior thereof. In the photoacoustic propagation medium 203, changes in the refractive index occur with propagation of the acoustic wave. By isolating these changes in the refractive index as an optical modulation by using a laser Doppler vibrometer 204, the acoustic wave 5 is detected. Patent Document 2 discloses that, by using a dry silica gel as the photoacoustic propagation medium 203, it is possible to introduce the acoustic wave in the waveguide into the photoacoustic propagation medium 203 with a high efficiency.